A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. A thin film transistor is used for a display device typified by a liquid crystal television device. A silicon-based semiconductor material is known as a semiconductor thin film that can be applied to a thin film transistor, and in addition, an oxide semiconductor has attracted attention as another material that can be applied to a thin film transistor.
As a material for the oxide semiconductor, zinc oxide and a material containing zinc oxide as its component are known. Further, a thin film transistor formed using an amorphous oxide (oxide semiconductor) having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Documents 1 to 3).